Vacuum-type circuit interrupter with means for reducing arc voltage during high instantaneous currents



PEA/f RC VOLT/IG- VOLT A Filed April 2o, 196e 3,321,599 MEANS FOR REDUCING VOLTAGE DURING HIGH INSTANTANEOUS CURRENTS 2 Sheets-Sheet l May 23. 1967 T. H. LEE VACUUM-TYPE CIRCUIT INTERRUPTER WITH Arron/viv T. H* LEE 3,321,599 TERRUPTER WITH MEANS FOR REDUCING G HIGH INSTANTANEOUS CURRENTS 2 Sheets-Sheet 2 /N vE/vro/Q.- THOMAS H. LEE, 5V 'g-LLM ITT/WVEYA Il I7 May 23, 1967 VACUUM-TYPE CIRCUIT IN ARC VOLTAGE DURIN Filed April 20, 1966 United States Patent Oiiice 3,321,599 Patented May 23, 1967 3,321,599 VACUUM-TYPE CIRCUIT INTERRUPTER WITH MEANS FOR REDUCING ARC VOLTAGE DUR- ING HIGH INSTANTANEOUS CURRENTS Thomas H. Lee, Media, Pa., assigner to General Electric Company, a corporation of New York Filed Apr. 20, 1966, Ser. No. 549,750 Claims. (Cl. 200-144) This application is a continuation-in-part of application S.N. 328,656, filed Dec. 6, 1963, and now abandoned.

This invention relates to a vacuum-type circuit interrupter for alternating currents and, more particularly, relates to a method and means for improving the current interrupting ability of such an interrupter.

The usual vacuum-type circuit interrupter comprises a pair of separable contacts or electrodes disposed within a vacuum chamber. Circuit interruption is initiated by separating these electrodes to establish a gap across which an arc is formed. The arc Vaporizes some of the electrode material to create a local atmosphere through which current flows until about the time a natural current zero is reached, assuming that the current being interrupted is an alternating current. After the current zero point has been reached, the usual recovery voltage transient begins building up across the gap between the electrodes. If the gap is able to withstand this recovery voltage transient, the arc is prevented from reigniting and the interruption is complete.

Whether the gap will be able to withstand the recovery voltageV transient is largely dependent upon the extent to which the gap is free of ionized arcing products when the recovery voltage transient is being applied. If, for example, the gap could be entirely freed of arcing products, the original vacuum, with its very high dielectric strength, would be available to withstand the recovery voltage transient. The extent to which the gap is free of ionized arcing products when the recovery voltage transient is applied depends to an important degree upon the ability of the nterrupter to condense these hot arcing products prior to this instant. The more completely the interrupter condenses the arcing products prior to this instant, the more likely it is that the gap will succeed `in Withstanding the recovery voltage transient.

During low current interruptions, the interrupter ordinarily has no diiiiculty in condensing the arcing products with suli'icient rapidity and completeness to withstand the recovery voltage transient. But, generally speaking, the higher the current being interrupted, the greater is the volume of arcing products generated, and the more difiicult it becomes to approach complete condensation of the arcing products in the available time.

Accordingly, an object of my invention is to improve the ability of the interrupter to condense the arcing products generated during high current interruptions.

Another object is to reduce the energy input into the interrupter during high current interruptions by means which does not impair the ability of the vacuum interrupter to rapidly recover its dielectric strength when the current reaches zero after a high peak current.

In carrying out my invention in one form, I provide an alternating-current vacuum interrupter that is adapted to interrupt currents having a peak value greater than 20,000 amperes. The interrupter comprises a pair of electrodes or contacts that have a separated position deiining a gap therebetween across which the arc is established. I apply to this gap an axial magnetic field that has its lines of force in the immediate region of the arc extending generally parallel to -the arc at substantially all locations occupied by the arc while in the aap. The magnetic tiel-d in the region of the arc is suiiiciently strong during high instantaneous currents, e.g., over 20,000 amperes, that it reduces the arc voltage to a Value substantially less than the arc voltage that would normally be present during corresponding values of instantaneous current without the axial magnetic field. As current zero is approached, the density of the magnetic eld is maintained suiiiciently low as to prevent substantial impairment of the voltage withstand ability of the gap at current zero as compared to that of a gap with no axial magnetic field during the period just preceding current zero.

For a better understanding of my invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. l is a cross-sectional view of a vacuum interrupter embodying one form of my invention.

FIG. 2 is a perspective View of a contact used in the interrupter ot FIG. l.

FIG. 3 is a graphical representation of the relationship between magnetic field strength and arc Voltage in an interrupter of the type shown in FIGS. l and 2.

FIG. 4 illustrates a moditied embodiment of the invention.

FIG. 5 is a cross sectional view of the interrupter of FIG. 4 taken along the line 5-5 of FIG. 4.

FIG. 6 is a graphical representation of certain electrical relationships present in the interruptor of FIG. 4.

Referring now to the interrupter of FIG. l, there is shown a highly evacuated envelope 10 comprising a casing 11 of suitable insulating material and a pair of metallic end caps 12 and 13 closing oft the ends of the casing. Suitable seals 14 are provided between the end caps and the casing to render the envelope vacuum tight. The normal pressure within the envelope 10 under static conditions is lower than l0h4 mm. of mercury, so that a reasonable assurance is had that the mean free path for electrons will be longer than the potential breakdown paths in the envelope.

Located within the envelope 10 is a pair of relatively movable disk-shaped contacts, or electrodes, 17 and 18 shown in their separated or open-circuit position. The upper contact is a stationary contact suitably secured to a conductive rod 17a, which at its upper end is united to the upper end cap 12. The lower contact 18 is a movable contact joined to a conductive operating rod 18a, which is suitably mounted for vertical movement. The operating rod 18a projects through an opening in the lower end cap 13, and a flexible metallic bellows 20 provides a seal about the rod 18a to allow for vertical movement of the rod without imnairing the vacuum inside the envelope 10. As shown in FIG. 1, the bellows 20 is secured in sealed relationship at its respective opposite ends to the operating rod 18a and the end cap 13.

Coupled to the lower end of the operating rod 18a, suitable actuating means t not shown) is provided for driving the movable contact 18 upwardly into en gaaement with the stationary contact 17 so as to close the interrunter. The closed position of the movable contact is indicated bv the dotted line 21. The actuating means is also capable of returning the contact 18 to its illustrated solid-line position so as to open the interrunter. A circuit opening oneration will soon be explained in greater detail. A typical gap length when the contacts are fully separated `is 1/2 inch.

The arc (indicated at 38) that is established across the gap 22 between the electrodes upon contact-separation vaporizes some of the contact material, and these vapors are dispersed from the arcing gap 22 toward the envelope. In the illustrated interrupter, the internal insulating surfaces of the casing 11 are protected from the condensation of arc-generated metallic particles thereon bv means of a tubular metallic shield 1S suitably supported on the casing 11 and preferably isolated from both end caps 12 and 13. This shield 15 acts to intercept and condense arc-generated metallic vapors before they can reach the casing 11. To 4reduce the chances for vapor bypassing the shield 15, a pair of end shields 16 and 16a are provided at opposite ends of the central shield. These end shields correspond to those disclosed and claimed in Patent No. 2,892,912, Greenwood et al., assigned to the assignee of the present invention.

All of the internal parts of the interrupter are substantially free of surface contaminants. In addition, the contacts 17 and 18 are effectively freed of gases absorbed internally of the contact body so as to preclude evolution of these gases during high current interruption.

Although this invention is not limited to any particular contact configuration, I prefer to use a contact configuration similar to that disclosed and claimed in U.S. Patent 2,949,520, Schneider, assigned to the assignee of the present invention. Accordingly, each contact is of a disk shape and has one of its major surfaces facing the other contact. The central region of each contact is formed with Va recess 29 in this major surface, and an annular contact-making area 30 surrounds this recess. These annular contact-making areas 30 abut against each other when the lower contact is in its dotted-line closed or engaged position and are of such a diameter that the current iiowing through the closed contacts follows a loop-shaped path L that bows radially outward, as is indicated by the dotted lines of FIG. l. This loop-shaped path has a magnetic effect which tends in a well known manner to lengthen the loop. As a result, when the contacts are separated to form an arc such as 38 between the areas 30, the magnetic effect of current fiowing through the loopshaped path wi-ll impel the arc radially outward.

As the arc terminals move toward the outer periphery of the disks 17 and 18, the arc 38 is subjected to a circumferentially-acting magnetic force that tends to cause the arc to move circumferentially about the central axis of the disks. This circumferentially-acting magnetic force is preferably produced by series of slots 32 provided in the disks and extending from the outer periphery of the disks radially inward by generally spiral paths, as is shown in FIG. 2. These 'slots 32 correspond to similarly designated slots in the aforementioned Schneider patent and, thus, force the current flowing to or from an arc terminal located at substantially any angular point on the peripheral region of the disk to follow a path that has a net component extending generally tangentially with respect to the periphery in the vicinity of the arc. This tangential configuration will be apparent from the dotted-line path L shown in FIG. 2 leading from rod 18a to t'he terminal of an arc 38 on the outer periphery of contact 18. This tangential configuration of the current path causes the magnetic loop L to develop a net tangential force component which tends to drive the arc in a circumferential direction about the contact.

As pointed out hereinabove, if the interrupter .is to successively interrupt the current at la given current zero, it must have built up suiiicient dielectric strength across the gap between the contacts to withstand the usual recovery voltage transient that appears across the contacts immediately following the -point at which current zero is reached. Whether or not the gap will have this much dielectric strength is largely dependent upon the extent to which the gap is free of arcing products by the time the recovery voltage transient is applied.

The extent to which the gap is free of arcing products depends to an important degree upon the ability of the interrupter, particularly the shield 1S, to condense these arcing products. Ordinarily, no problem is encountered for `low :current interruptions since the quantity of arcing products generated by a low current arc is relatively small. But at high currents, much greater quantities of arcing products are generated, and there is a current level beyond which the interrupter can no long-er condense these `arcing products fast enough for the gap to withstand the recovery voltage transient. In the interrupter of FIG. l,

I have materially improved the ability of the interrupter to condense the arcing products generated during high current interruptions by applying an axial magnetic field 50 to the arcing gap 22 between the contacts, As will soon be explained in detail, this axial magnetic field is controlled in such a manner that it is strong during the period when the arcing .current is high and is very weak during current Zero and the period just preceding current zero.

This axial magnetic field is produced by a coil 52 that encircles the cylindrical insulating casing 11 and is connected in series with the power circ-uit through the interrupter so that the current flowing through the arc also flows through the coil 52. During arcing, the circuit through the interrupter and the coil 52 extends between a pair of opposite terminals 54 and 56 via the rod 18a, contact 18, the arc 38, contact 17, rod 17a, connection 57 and coil 522. When current fiows through the coil 52, it creates the magnetic field 50 that has its lines of force extending lgenerally parallel to the arc 38 in the arcing gap 22. Since these lines of force, in extending generally parallel to the arc, also extend `axially of the arc, the magnetic field is referred to as an axial field. At any arcing location in the gap 22, the lines of force 50 in the region of the arc extend generally parallel to the longitudinal axis of the arc.

I have found that the application of a strong axial magnetic field to the arcing gap of a vacuum interrupter can very materially reduce the arc voltage developed during high instantaneous currents. The energy input into the shield during the arcing period varies as a direct function of the arc voltage. Hence, by reducing this `arc voltage at high currents, I can reduce the energy input into the shield. This enables me to limit the temperance rise of the shield, thus preserving its ability to rapidly condense the vapors that are released from the arcing gap.

In a series of tests made with an interrupter similar to that shown in FIG. l, except with the coil 52 separately energized from a D.C. source, it was found that the peak arc voltage could be drastically reduced with magnetic fields of quite moderate densities. In each of these tests, .approximately a half cycle of arc current with a given peak value was passed through the interrupter while an axial magnetic field of a given density was maintained. The approximate peak arc voltages developed during these tests are shown plotted in the solid line curves of FIG. 3. The dotted-line curve of FIG. 3 designated no field was obtained by passing current directly through the interrupter with coil 52 not present in the circuit. The -arc voltage developed under these conditions is referred to as that normally developed when no axial field is present. FIG. 3 shows that with a magnetic field of 6,000 gausses and peak currents in the neighborhood of 50,000 to 60,000 amperes, the arc voltage can be drastically reduced to about one-third of that which would normally be present without the magnetic field. With lower field strengths when the peak current is in the 50,000 to 60,000 ampere range, the reduction in arc voltage is not as great, but even with a field strength of 1,600 gausses, the reduction in arc voltage is almost as great as that obtained with 6,000 gausses. From the curves of FIG. 3, it will be apparent that the field strength required for a given reduction in arc voltage incre-ases as the peak arc current increases. lt will also be apparent that for -a given value of current, there is a field strength above which increased field strength has little effect in further reducing arc voltage. For example, in the curve of FIG. 3 it will be observed that between 40,000 'and 50,000 amperes, an increase `in magnetic field beyond 1,600 gausses produces little or no further reduction in are voltage.

At currents below about-8,000 to 10,000 amperes, an axial magnetic field, even a very strong one, appears to produce little or no reduction in arc voltage. This will be apparent from the left hand end of the curves of FIG. 3.

In a preferred embodiment of my invention, the strength of the magnetic field 50 is made high enough to reduce the -arc voltages developed during high peak currents to less than half the arc voltage that would normally be developed without the axial field. But in its broader aspects, the invention is not so limited. For example, if higher arc voltages can be tolerated, then reduce-d field strengths may be employed. In any case, however, I use field strengths great enough to produce a substantial reduction (i.e., greater than about 25 percent) in arc voltage for instantaneous currents above 20,000 amperes. By way of example, in one embodiment of my invention, I provide a field-producing arrangement that can develop in the arcing gap 800 gausses at 20,000 amperes instantaneous current, 1,600 gausses at 40,000 amperes, and 2,400 gausses at 60,000 amperes. The approximate arc voltage that will be developed for a given instantaneous current with a given field present can be determined from the curves of FIG. 3. Even though the data of FIG. 3 was obtained using a D.C. field, it is directly applicable to the interrupters of FIGS. 1 and 4 since it is the instantaneous field strength that determines the arc voltage for a given current.

To increase the current interrupting capacity of a vacuum interrupter, it is not enough merely to maintain across the gap a strong axial magnetic field that -reduces the peak arc voltages. More specifically, I have found that the presence of a strong axial magnetic field during the period. when the alternating current is approaching zero can seriously impair the current interrupting ability of the vacuum interrupter even though the arc voltage during high values of instantaneous current had been reduced. Thus, in a preferred form of my invention, I control the axial magnetic field in such a manner that it is substantially or effectively eliminated during current zero and the period immediately preceding it. It is not knecessary to completely eliminate the axial magnetic field during this crucial period, but its field strength should be reduced to a sufficiently low value that there is no substantial impairment of the recovery voltage withstand ability of the arcing gap as compared to that of the gap without an axial magnetic field during this interval around current zero.

Since the coil 52 of FIG. 1 is in series with the contacts 17, 18 of the interrupter, it will be apparent that the current through the coil and the arcing current are in phase. This, however, does not automatically assure that the magnetic field or flux produced by the coil 52 in the arcing gap will be in phase With the arcing current. Unless the eddy currents induced by the magnetic field are held to a low value, then they will cause an appreciable lag in the flux behind the current, and this will result in a relatively high magnetic field remaining across the gap when the current zero point is approached. To reduce these eddy currents to such a level that the flux and the arcing current are approximately in phase, I form the end plates of a high resistivity, low permeability material, `such as stainless steel, and I utilized the slots 32 in the contacts 17 and 18 to break up the eddy current paths through the contacts. With regard to this latter feature, I extend the slots 32 radially inward as far as possible so as to improve their effectiveness in breaking up the eddy current paths. In addition, each contact is perforated in its central region 29, as shown at 70 in FIG. 2, to further reduce the eddy currents. With the eddy currents so reduced, I can maintain the flux sufficiently in phase with the arcing current t provide a loW enough field strength at and just before current zero to prevent any substantial impairment of the gaps recovery voltage withstand ability at current zero.

In terms of its physical effect on the arc, it appears that the axial magnetic field, when its strength is high, prevents the arc from bowing and also confines the arcing products about the arc column. These effects account for the above described reduction during arc voltage in high currents. On the other hand, when the field strength is very low, the field has no confining effect upon the arcing products, and they are free to disperse from the arcing gap. Thus, by substantially or effectively eliminating the axial magnetic field when the arcing current approaches zero, the arcing products are permitted to disperse from the gap in time for the gap to recover its dielectric strength sufficiently to withstand the usual recovery voltage that appears immediately after current zero is reached.

FIG. 4 shows a vacuum interrupter that is provided with additional means for controlling the density of the axial magnetic field in the arcing gap. The arrangement of FIG. 4 is considered to fall within the scope of my invention, but its specific features are shown and claimed in application Ser. No. 328,601, Greenwood and Porter, led Dec. 6i, 1963 now Patent No. 3,283,103, and assigned to the assignee of the present invention.

From a structural viewpoint, this arrangement of FIG. 4 differs from that of FIG. 1 principally in its inclusion of an iron core l60 between the coil 52 and the insulating casing 11. This iron core 60 is made of a high permeability material such as silicon steel. It is formed from strips of grain-oriented silicon steel arranged in circumferentially-spaced stacks 62 about the interrupter casing as best shown in FIG. 5. The stacks 62 are held in assembled relationship by suitable means comprising a cylin* der 63 of insulating material.

When the current through the interrupter and the series connected coil 52 is low, the iron core 60 is unsaturated and thus acts as a fiux shunt through which most of the fiux -developed by coil 52 is directed so that very little flux penetrates into the arcing gap. When the current through the coil 52 rises to a high value, the core saturates at a predetermined current level and thus becomes ineective to act as a flux shunt for flux produced by current in excess of said predetermined level. A high percentage of this fiux thus penetrates into the arcing gap and produces an axial field of high `density in the arcing gap during high instantaneous currents.

This relationship is illustrated in FIG. 6 where curve F depicts the fiux iu the center of the contact region during a period of high current such as might result from a short circuit. Such current is depicted in curve I, plotted against the same time scale as the curve F. The current of curve I is depicted as fiowing for a complete half cycle from O to C. Between the instants O and A, the instantaneous current is relatively low and the iron core `60 is unsaturated. Thus, most of the flux is directed through the core 60, and very little penetrates into the contact region, as is indicated by the relatively fiat portion of the fiux curve F between O and A. Following the instant A, the core 60 begins saturating and the fiux created by the additional current can no longer find a loW reluctance path through the core. Accordingly, a high percentage of this flux penetrates into the contact region, causing the flux curve F to rise at a much steeper rate. Shortly after the current reaches its peak, the fiux also reaches its peak and then drops as the current drops. At the instant B, the current has dropped to a level that has restored the iron to its unsaturated condition, thus allowing the iron to shunt most of the fiux through a path remote from the contact region. Some stray flux continues to appear in the con-tact region after the instant B, but this is a relatively small amount of flux, as illustrated by the low fiat portion of the fiux curve F extending from B to C.

In a typical interrupter constructed as shown in FIGS. 4 and 5, the iron core 60 was designed to saturate at approximately 23,000 amperes. For instantaneous currents above 23,000 amperes, an axial field appeared in the contact region high enough to reduce .the arc voltage to a value less than half that which typically or normally appeared without the axial magnetic field. With a circuit voltage of 15.5 kv. R.M.S., asymmetrical currents with peak values as high as 65,000 amperes were interrupted by this interrupter. This 65,000 ampere peak current was approximately 50% higher than the maximum peak current that could typically be interrupted by interrupters of this same design, but without the axial magnetic field and the iron core. In the arrangement of FIGS. 4 and 5, a small amount of flux appears in the arcing 4gap between the instants B and C, but the density of this magnetic field is kept low enough to prevent any substantial impairment in the voltage withstand ability of the gap as compared to this gap with no axial magnetic field during .the interval immediately preceding current Zero. For example, it has been found that the arrangement of FIGS. 4 and 5 can limit the strength of the magnetic field remaining in the arcing gap at current zero to less than 100 gausses even after peak currents that developed 2,000 and 3,000 gausses. A residual magnetic field of such strength at and immediately before current zero does not substantially impair the voltage withstand ability of the gap. Since this slight residual field does not substantially irnpair the voltage withstand ability of the gap at current zero, the axial magnetic field can be considered as effectively eliminated just prior to and upon reaching current zero.

If the ux wave form had been approximately the same as that of the current, it will be apparent that the amount of flux at instant B would be a higher percent-age of the maximum flux than is the case with the flux wave form F shown in FIG. 6. Thus, the presence of the core 60 permits a reduction in the amount of ux appearing just before current zero for a given maximum Value of liux. This facilitates production of the desired high amount of flux in the arcing gap during high currents without producing excessive flux during the period just prior to current zero.

In the interrupter of FIGS. 4 and 5, eddy currents are reduced in the same manner as described in connection with the interrupter of FIGS. l and 2. By thus reducing the eddy currents, the lag of the flux behind the current is limited to such an extent that only a small amount of flux remains in the arcing gap at current zero, as was explained in more detail with respect to FIGS. l and 2.

While I have .shown and described particular embodiments of my invention it will Ibe `obvious to those skilled in the art that various changes and modifications may be rnade without departing from my invention in its lbroader aspects, and I, therefore, intend in Ithe appended claims to cover all such changes and modifications as fall Within lthe true spirit and scope or my invention.

What I claim as new and desire to .secure by Letters Patent of the United States is:

1. An alternating current circuit interrupter of the vacuum type that is yadapted to interrupt currents having maximum instantaneous values higher than 20,000 arnperes comprising:

(a) a highly evacuated envelope,

(b) a pair of electrodes within said envelope having a spaced-apart position and defining an arcing gap therebetween across which an alternating current arc is established,

(c) means developing in the immediate region of an larc at substantially any arcing location in said gap an axial magnetic field having its lines of force extending between said electrodes via pat-hs generally parallel to the longitudinal axis of the are,

(d) means controlling said magnetic field so that its fiux density in the region of the arc during instan- Itaneous arcing currents greater than 20,000 amperes will be high enough to substantially reduce the arc voltage as compared to that are voltage normally developed during corresponding instantaneous currents without said `axial magnetic field,

(e) means including eddy-current-reducing means effectively leliminating said axial magnetic field during the period just prior and upon reaching to a current zero following an instantaneous arcing current greater than 20,000 amperes so that there is no substantial impairment of the voltage withstand ability of said gap as compared to a gap having no axial field present during said period.

2. The interrupter of claim 1 in which said axial magnetic field has a high enough density for instantaneous currents above 40,000 amperes Vto lreduce the instantaneous arc voltage dor such currents to less than half the arc voltage normally developed during corresponding instantaneous `currents when no axial magnetic field is present.

3. An alternating current circuit interrupter of the vacuum type that is adapted to interrupt currents having maximum instantaneous values higher than 20,000 amperes comprising:

(a) a highly evacuated envelope,

(b) a pair orf electrodes within said envelope having a spaced-apart position and defining an arcing gap therebetween across which an alternating current arc is established,

(c) means developing in the immediate region of an arc at substantially any arcing location in said gap an axial magnetic field having its lines of force extending between said electrodes via paths generally parallel to the longitudinal axis of the arc,

(d) means controlling said magnetic field so that its flux density in the region of the arc during instantaneous arcing currents greater than 20,000 amperes will be high enough to substantially reduce the arc voltage as compared to that arc voltage normally developed during corresponding instantaneous currents without said axial magnetic field,

(e) means including eddy-current-reducing means effectively eliminating said magnetic field during the period just prior to and upon reaching a current zero following an instantaneous arcing current greater than 20,000 amperes.

4. The interrupter of claim 3 in which said axial magnetic field has a high enough density for instantaneous currents above 40,000 amperes to reduce the instantaneous a-rc voltage for such currents to less than half the arc voltage normally developed during corresponding instantaneous currents when no axial magnetic eld is present.

5. The circuit interrupter of claim 3 in which said electrodesV are relatively-movable engageable electrodes that are movable out of engagement to establish said arcing gap.

6. An alternating current circuit interrupter of the vacuum type that is adapted to interrupt currents having maximum instantaneous values higher than 20,000 amperes comprising:

(a) a highly evacuated envelope,

(b) a pair of electrodes within said envelope having a spaced-apart position and defining an arcing gap therebetween across which an alternating current arc is established and caused to burn,

(c) means developing across said gap an axial magnetic field having most of its lines olf rforce that are located within said gap extending between said electrodes generally parallel to the longitudinal axis of said are, said generally parallel relationship being maintained as the arc continues to burn across said gap during the passage of high instantaneous currents through the arc,

(d) means controlling said magnetic field so that its linx/density in the region of the arc during instantaneous arcing currents greater than 20,000 amperes will be high enough to substantially reduce the arc voltage as compared to that arc voltage normally developed during corresponding instantaneous currents without said axial magnetic field,

(e) means including eddy-current-reducing means effectively eliminating said axial magnetic eld during the period just prior to and upon reaching a cur- 'rent zero following an instantaneous arcing current greater than 20,000 atmperes so that there is no substantial impairment of the voltage withstand ability of said gap as compared to a gap having no axial iield present during said period.

7. An alternating current circuit interrupter of the vacuum type that is adapted to interrupt currents having maximum instantaneous values higher than 20,000 anrperes comprising:

(a) a highly evacuated envelope,

(b) a pair of electrodes within said envelope having a spaced-apart position and defining an arcing gap therebetween across which an alternating current arc is established and caused to burn,

(c) means developing across said gap an axial magnetic iield having most of its lines of force that are located within said gap extending between said electrodes generally parallel to the longitudinal 'axis of said are, said generally parallel relationship being maintained as the arc continues to burn across said gap during the passage of high instantaneous currents through the arc,

(d) means controlling said magnetic lield in such a manner that its iiux density in the region of the arc during instantaneous arcing currents greater than 20,000 amperes will be high enough to substantially reduce the arc voltage as compared to that yarc voltage normally developed during corresponding instantaneousl currents without said axial trnagnettic field, land (e) means for reducing the density of said axial magnetic eld during the period just prior to and upon reaching 'a current zero following an instantaneous arcing current greater than 20,000 amperes to such a level that there is no substantial impairment of the voltage withstand ability of said gap as compared to a gap having no axial yfield present during said period,

(f) said means for reducing the density of said axial magnetic field comprising eddy-current-reducing means for breaking up the paths of eddy currents induced in the portions of said electrodes adjacent said arcing gap.

8. An alternating current ycircuit interrupter of the vacuum type that is adapted to interrupt currents having maximum instantaneous values higher than 20,000 arnperes comprising:

(a) a highly evacuated envelope,

(b) a pair of electrodes within said envelope having a spaced-apart position and defining an arcing gap therebetween across which an alternating current arc is established and caused to burn,

(c) means developing across said gap an axial magnetic field having most of its lines of force that are llocated within said gap extending between said electrodes generally parallel to the longitudinal `axis of said arc, said generally parallel relationship being maintained as the arc continues to burn across said gap during the passage of high instantaneous currents through the arc,

(d) means controlling said magnetic field in such a manner that its linx density in the region of the arc during instantaneous arcing currents greater than 20,000 amperes Will ybe high enough to substantially reduce the are voltage as compared to that arc voltage normally developed during corresponding instantaneous currents without said axial. magnetic iield, and

(e) means for reducing the density of said axial magnetic field during the period just prior to and upon reaching a current zero following an inst-antaneous arcing current greater than 20,000 amperes to such a level that there is no substantial impairment of the voltage withstand ability of said gap as compared to a gap having no axial iield present during said period,

(f) said axial magnetic field having a liux density of greater than 800 gausses in the region of the arc during instantaneous arcing currents of 40,000 amperes, and said axial magnetic iield having a density of lless than 200 gausses just prior to current zero following an instantaneous arcing current greater than 20,000 amperes.

9. The circuit interrupter of claim 6 in combination with a tubular metal shield spaced from said arcing gap and arranged to intercept and condense metal vapors emitted from said arcing gap.

10. The circuit interrupter of claim 1 in combination with a tubular metal shield spaced from said arcing gap and arranged to intercept emitted from said arcing ga References Cited by the Examiner ROBERT K. SCHAEFER, Primary Examiner. R. S. MACON, Examiner.

and condense metal vapors' 

1. AN ALTERNATING CURRENT CIRCUIT INTERRUPTER OF THE VACUUM TYPE THAT IS ADAPTED TO INTERRUPT CURRENTS HAVING MAXIMUM INSTANTANEOUS VALUES HIGHER THAN 20,000 AMPERES COMPRISING: (A) A HIGHLY EVACUATED ENVELOPE, (B) A PAIR OF ELECTRODES WITHIN SAID ENVELOPE HAVING A SPACED-APART POSITION AND DEFINING AN ARCING GAP THEREBETWEEN ACROSS WHICH AN ALTERNATING CURRENT ARC IS ESTABLISHED, (C) MEANS DEVELOPING IN THE IMMEDIATE REGION OF AN ARC AT SUBSTANTIALLY ANY ARCING LOCATION IN SAID GAP AN AXIAL MAGNETIC FIELD HAVING ITS LINES OF FORCE EXTENDING BETWEEN SAID ELECTRODES VIA PATHS GENERALLY PARALLEL TO THE LONGITUDINAL AXIS OF THE ARC, (D) MEANS CONTROLLING SAID MAGNETIC FIELD SO THAT ITS FLUX DENSITY IN THE REGION OF THE ARC DURING INSTANTANEOUS ARCING CURRENTS GREATER THAN 20,000 AMPERES WILL BE HIGH ENOUGH TO SUBSTANTIALLY REDUCE THE ARC VOLTAGE AS COMPARED TO THAT ARC VOLTAGE NORMALLY DEVELOPED DURING CORRESPONDING INSTANTANEOUS CURRENTS WITHOUT SAID AXIAL MAGNETIC FIELD, (E) MEANS INCLUDING EDDY-CURRENT-REDUCING MEANS EFFECTIVELY ELIMINATING SAID AXIAL MAGNETIC FIELD DURING THE PERIOD JUST PRIOR AND UPON REACHING TO A CURRENT ZERO FOLLOWING AN INSTANTANEOUS ARCING CURRENT GREATER THAN 20,000 AMPERES SO THAT THERE IS NO SUBSTANTIAL IMPAIRMENT OF THE VOLTAGE WITHSTAND ABILITY OF SAID GAP AS COMPARED TO A GAP HAVING NO AXIAL FIELD PRESENT DURING SAID PERIOD. 